Method for the production of cross-linkable acrylate contact adhesive materials

ABSTRACT

The invention relates to a method for the production of cross-linked acrylate contact adhesive materials characterised in that polyacrylates are firstly produced from the following monomers by free-radical (co-)polymerisation: (d) acrylic and methacrylic acid monomers with the following structure, where R 1 ═H or CH 3  and R 2 =an alkyl chain with 2-20 C atoms, at a proportion of 45 99.5 wt. % of the monomer mixture, (e) one or several carboxylic acid anhydrides, containing olefinic double bonds, at a proportion of 0.5-25 wt. % of the monomer mixture, (f) further unsaturated olefinic monomers containing functional group A, at a proportion of 0-30 wt. % of the monomer mixture. The polymers thus produced are concentrated to give a polyacrylate material with a solvent content ≦2 wt. %, further monomers are added to the polyacrylate material, which contain at least two functional groups B and C, whereby the group B can undergo polymerisation-type reactions with the carboxylic acid anhydrides and the groups C are cross-linkable groups. A reaction between the functional groups B and the carboxylic acid anhydrides occurs, which links the monomer containing the functional group B to the polymer in the form of a sidechain. After the reaction between the functional group B and the carboxylic acid anhydride a cross-linking of the polymers is carried out by means of an energetic irradiation.

[0001] The invention relates to a process for preparing polyacrylateswhich are functionalized with double bonds and have pressure-sensitivelyadhesive properties, whose cohesion is increased by radiation-inducedcrosslinking, and to an adhesive tape provided with this polyacrylatepressure sensitive adhesive.

[0002] Hotmelt pressure sensitive adhesives (hot melt PSAs) arecompounds which combine the properties of hotmelt adhesives with thoseof pressure sensitive adhesives. Hotmelt PSAs melt at elevatedtemperatures and cool to form a permanently tacky film which flowsadhesively on contact with a substrate. In combination with varioussubstrates, such as paper, fabric, metal, and polymer films, forexample, it is possible to produce a large number of different products,particularly pressure sensitive adhesive tapes and also labels. Thesepressure sensitive adhesive products have a broad field of applicationin the automobile industry, e.g., for fastening or for sealing, or inthe pharmaceutical industry, for active substance patches, for example.

[0003] The typical coating temperature for hotmelt PSAs lies between 80and 180° C. In order to minimize the coating temperature, the molecularweight of the hotmelt PSA to be applied should be as low as possible. Onthe other hand, the PSA must also possess a certain level of cohesion,so that the PSA tape does not slip from the substrate in use. In orderto increase the cohesion, in turn, a high molecular weight is essential.

[0004] In order to solve this problem polymers have been developed whichpossess a relatively low molecular weight but contain double bonds alongthe side chains. These polymers, such as polyester acrylates orpolyurethane acrylates, for example, can be crosslinked efficiently viathe double bonds using UV or ionizing radiation, but have only limitedadhesive properties.

[0005] In acrylic PSAs, crosslinking is promoted by addingpolyfunctional acrylates and/or methacrylates prior to crosslinking,which raise the crosslinking reactivity and hence also increase thecohesion, but which react only by way of a two-stage mechanism duringirradiation (attachment to the polymer and then crosslinking reaction byway of the acrylate double bond which is still free) and therefore havea low crosslinking efficiency.

[0006] The principle of functionalizing double bonds by copolymerizationcannot be employed analogously for acrylic PSAs, since in that case thecorresponding polyacrylates are prepared by free radical polymerization.All of the double bonds here are reacted in the polymerization process,or instances of gelling occur during polymerization. One example of thiswas depicted by Pastor [U.S. Pat. No. 4,234,662 A], who used allylacrylate or allyl methacrylate for the polymerization. A centralproblem, however, lies in the copolymerization of these compounds, whichgenerally gel during the free radical polymerization process. Moreover,owing to the relatively low reactivity of the allyl groups in respect ofa crosslinking reaction, drastic experimental conditions are necessary,in particular high temperatures or a long period of irradiation. Forapplication as a crosslinked PSA, therefore, the allyl-modified acrylicpolymers are not very suitable.

[0007] Another possibility for functionalization of double bonds existsby virtue of polymer-analogous reactions.

[0008] Generally speaking, polymer-analogous reactions can be conductedin solution or from the melt. EP 0 608 981 B1 likewise refers to thegelling problem with double bonds. This is assisted by diverse furtherpolymer-analogous reactions. Accordingly, polyacrylates with carboxylicacid, hydroxyl, epoxide, and amine groups can be reacted in apolymer-analogous reaction with compounds containing double bonds; inthis regard see U.S. Pat. No. 4,665,106 A. Owing to the low thermalstability of the components involved, however, it has not been possibleto apply this reaction to hotmelts. Moreover, operating conditions weredisadvantageous owing to the fact that in order to avoid gelling it wasnecessary to add large amounts of regulator to the polyacrylate.

[0009] For acrylic hotmelts, therefore, U.S. Pat. No. 5,536,759 Adescribes the reaction of polyacrylates containing hydroxyl orcarboxylic acid groups with1-(1-isocyanato-1-methylethyl)-3-(1-methylethenyl)benzene (m-TMI) insolution with subsequent hotmelt processing. It is in contrast to thisthat the pros and cons of the individual processes are described (ChemieIngenieur Technik (70), 1998, pp. 560-566); “Polymer-analogous reactionsin the melt permit two processes which otherwise proceed separately fromone another. First of all, the reaction takes place; since the reactionmedium is the melt, shaping by extrusion can be commenced during thereaction. In this way, no additional reaction vessel and no work-up atall are necessary. Nevertheless, the absence of the solvent complicatesthe course of the reaction in a variety of respects; for example, by theheterogeneity of the reaction mixture and the relatively slow diffusionof the reactants into one another”.

[0010] Accordingly, the process described in EP 0 608 981 B1 displaysthe fundamental disadvantages of a polymer-analogous reaction insolution. What would be desirable, therefore, would be a process foracrylic PSAs which allows polymer-analogous reactions in the melt.

[0011] A central problem lies in the slow diffusion of the reactants.This problem can be solved only by raising the reaction temperatures,which improves the reactivity of the individual components with oneanother. For acrylic PSAs, however, there are natural limits on this.

[0012] For polymer-analogous reactions in the melt, therefore, thematerials used are generally thermoplastics, which are processed andfunctionalized at high temperatures. For example, polystyrene-maleicanhydride thermo-plastics are reacted at temperatures of 180-200° C.[Chemie Ingenieur Technik (70), 1998, pp. 560-566 and Chemie IngenieurTechnik (71), 1999, pp. 1418-1421]. Additionally, polyesters are reactedwith maleic anhydride in the melt [Journal of Polymer Science: Part A:Polymer Chemistry, Vol. 37, 1693-1702 (1999)]. Both processes, however,are unsuitable for the functionalization for acrylic PSAs with doublebonds. Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 37,1603-1702 (1999) discloses functionalization by radical grafting, butthis cannot be used for functionalization with double bonds on accountof the fact that the vinyl compounds would immediately be polymerizedand so would no longer be available for subsequent crosslinking on thebacking. The prior art uses polymers [Chemie Ingenieur Technik (70),1998, pp 560-566] whose glass transition temperatures are too high forPSAs and, if applied analogously to acrylic PSAs, would exhibitexcessively high reaction temperatures (at the high temperaturesemployed, severe discoloration of the polymer already occurs, owing toreactions by, for example, thermally decomposing initiators which haveremained after the polymerization process or by the decomposition ofindividual copolymers, such as tert-butyl acrylate, for example, atabove 160° C., and also possess very high fractions of copolymerizedmaleic anhydride, which places the glass transition temperature veryhigh. The process described in U.S. Pat. No. 5,536,759 A for reactingpolyacrylates with1-(1-isocyanato-1-methylethyl)-3-(1-methylethenyl)benzene (m-TMI) insolution can also not be applied analogously for the processes describedabove, since in addition to the high toxicity of the isocyanates thecrosslinking reactivity after coating would be too low.

[0013] A serious and general disadvantage of all of the processesdescribed so far lies in the low crosslinking reactivity after coating.Vinyl compounds have a low reactivity toward the radicals that aregenerated for crosslinking, with the consequence that crosslinking isincomplete and not very effective. Competing reactions which do not leadto the desired crosslinking, such as saturation of the radicals producedby atmospheric oxygen or added tackifier resins, predominate. The poorcontrollability of crosslinking may therefore be very problematic, forexample, for the aging behavior of the PSA, since unless all of thedouble bonds are consumed by reaction during crosslinking, the PSA willhave a potential for post-crosslinking on prolonged storage and also,under the influence of ultraviolet light or oxygen and/or ozone, willreact and undergo a marked loss of bond strength.

[0014] There is therefore a need for compounds which can be reacted veryquickly and without gelling in a polymer-analogous reaction and for aprocess operation which allows gel-free processing and coating on abacking, with the aim of obtaining new kinds of acrylic PSA tapesfunctionalized by reactive double bonds, which can be crosslinked withhigh reactivity by actinic radiation.

[0015] It is an object of the invention to provide a process forpreparing pressure sensitive adhesives on an acrylic basis which have aviscoelastic behavior at room temperature and which do not show thedisadvantages of the prior art. The aims are to prevent gelling of thePSAs during the hot melt process; in particular, not to lose the thermalstability of the PSA as a result of incorporation of the double bonds;that it should be possible to subsequently coat the acrylic PSA providedwith reactive double bonds from the melt onto a backing without gel; andthat said PSA should be crosslinked with high crosslinking efficiency.

[0016] This object is achieved, surprisingly and unforeseeably for theskilled worker, by a process as set out in the main claim. The furtherclaims relate to advantageous developments of this process, to thepressure sensitive adhesive prepared thereby, and to a use for saidadhesive.

[0017] The invention accordingly provides a process for preparingcrosslinked acrylic pressure sensitive adhesives which involves

[0018] first preparing polyacrylates by free-radical (co)polymerizationfrom the following monomers:

[0019] (a) acrylic and methacrylic acid monomers of the followingstructure:

[0020] where R₁H or CH₃

[0021] and R₂ an alkyl chain with 2-20 carbon atoms

[0022]  with a fraction of 45-99.5% by weight in the monomer mixture,

[0023] (b) one or more carboxylic anhydrides containing olefinic doublebonds,

[0024] with a fraction of 0.5-25% by weight in the monomer mixture, morepreferably with a fraction of 1-5% by weight in the monomer mixture,

[0025] (c) further olefinically unsaturated monomers possessingfunctional groups A,

[0026] with a fraction of 0-30% by weight in the monomer mixture

[0027] concentrating the polymers thus prepared to give a polyacrylatecomposition whose solvent content is ≦2% by weight,

[0028] adding further monomers to the polyacrylate composition, thesemonomers possessing at least two functional groups B and C, the groups Bbeing able to enter into polymer-analogous reactions with the carboxylicanhydrides, and the groups C being crosslinkable groups,

[0029] a reaction taking place between the functional groups B and thecarboxylic anhydride, which attaches the monomers containing functionalgroups B as side chains to the polymers,

[0030] following the reaction between the functional groups B and thecarboxylic anhydride, applying the pressure sensitive adhesive from themelt to a backing, and

[0031] carrying out crosslinking of the polymers on the backing by meansof high-energy radiation.

[0032] The process makes it possible to introduce into the polymersfunctional groups which are available for efficient crosslinking lateron under mild conditions, without the functional groups which permitcrosslinking being consumed or losing their functionality during thepolymerization process.

[0033] As a result of the introduction of the compounds containing thefunctional groups B and C after polymerization has already taken place,the functional groups C which are reactive for crosslinking retain theirreactivity even following incorporation into the polymer chains.Functional groups which possess a high efficiency for the crosslinkingreaction can be introduced into the polymers in this way, even though inthe case of a free-radical polymerization these functional groups wouldlose their functionality.

[0034] The average molecular weights (weight average Mw) of the PSAswhich form in the course of the free-radical polymerization are chosensuch that they are situated within a range which is customary forpolyacrylate compositions, i.e., between 100 000 and 2 000 000;specifically for further use as hotmelt PSAs, PSAs having molecularweights (weight average M_(w)) of from 100 000 to 800 000, morepreferably from 100 000 to 400 000, g/mol are prepared. Thepolymerization may be conducted in the presence of an organic solvent,in the presence of water or in mixtures of organic solvents and water.The aim is to minimize the amount of solvent used. Depending onconversion and temperature, the polymerization time is between 6 and 48hours. The higher the reaction temperature which can be chosen, i.e.,the higher the thermal stability of the reaction mixture, the shorterthe reaction time that can be chosen.

[0035] For the purposes of the invention it is particularly advantageousto operate the process such that the addition of the monomers possessingthe functional groups B and C and the reaction of the functional groupsB with the carboxylic anhydride take place directly after theconcentration step.

[0036] In one development of the invention, this operation takes placein an extruder: a twin-screw extruder (e.g. Werner & Pfleiderer, ZSK 40)or a co-kneader (e.g. Buss) have been found highly suitable for thispurpose (reactive extrusion). In the extruder, the acrylic PSAs preparedby free-radical polymerization are concentrated and freed from thesolvent. Advantageously for the process of the invention, the solventcontent of the polymer composition following the concentration operationis less than 0.5% by weight. Following concentration the componentprovided with the functional groups B and C is added to the extruder orto the co-kneader, preferably by metering. Here, the reaction takesplace between the functional groups B and the carboxylic anhydridegroups incorporated into the polymer chains. In one preferred embodimentof the inventive process, the addition may be made in a second extruder.In this case the optimum reaction conditions can be set by means ofbarrel length, throughput (rotary speed), kneading temperature, andamount of any catalyst added. Moreover, a relatively low-shear screwgeometry of the extruder should be chosen in order to prevent instancesof gelling during operation.

[0037] Advantageously for the purposes of the invention it is possibleto use any compounds which contain crosslinkable groups C and which alsopossess a hydroxyl function that can react with the carboxylicanhydride.

[0038] As well as compounds substituted by hydroxyl groups, it ispossible to make favorable use, for the inventive process, of compoundscontaining crosslinkable groups C and also containing other functionalgroups which are able to react, directly or under catalysis, with thecarboxylic anhydride, particularly in a linking reaction. Suchfunctional groups are familiar to the skilled worker; here, mention maybe made, by way of example and without wishing to be unnecessarilyrestricted by their listing, of the following: alkoxy groups, mercaptogroups, thioether groups, hydroxyl groups, unsubstituted and substitutedamino groups, oxazolines and/or unsubstituted or substituted amidogroups, as well as all other functional groups which are reactive withcarboxylic anhydrides in the sense outlined above.

[0039] In order to ensure good and efficient crosslinking of thepolymers, examples of crosslinkable groups B used to outstanding effectare vinyl groups or, even more preferably, acrylate or methacrylategroups, which may also be in the form of their substituted derivatives.Accordingly, monomers containing functional groups B and C which areadvantageous in the sense of the inventive concept arehydroxyl-containing acrylates, such as, very preferably, for example,2-hydroxyethyl acrylate (2-HEA, acrylic acid 2-hydroxyethyl ester),hydroxypropyl acrylate (acrylic acid 3-hydroxypropyl ester), andhydroxyl-containing methacrylates, such as 2-hydroxyethyl methacrylate(2-HEMA, methacrylic acid 2-hydroxyethyl ester), hydroxypropylmethacrylate (methacrylic acid 3-hydroxypropyl ester), for example,and/or vinyl compounds, such as 1-decenol, for example, oxazolines, suchas ricinene-alkyloxazoline or soyaalkyloxazoline, for example,acrylamides, such as butoxymethylacrylamide, for example, or substitutedamino compounds, such as tert-butylaminoethyl methacrylate, for example.

[0040] The molar fraction of the compound added which is functionalizedby the groups B and C corresponds preferably to the molar amount of thecarboxylic anhydride which is incorporated by polymerization in thepolyacrylate chain, but may also be chosen to be smaller or larger thanthat amount.

[0041] The amount of the compound functionalized by the groups B and Cwhich is added is very preferably chosen such that the molar ratio ofthe number n_(B) of functional groups B of the added monomers to thenumber n_(CSA) of the copolymerized carboxylic anhydride unitsn_(B)/n_(CSA), is within a range of magnitude of between 0.8 and 1.2,very preferably between 0.8 and 1, i.e., nB/nCSA is <1.

[0042] In one preferred variant of the inventive process between 0.1 and25%, preferably between 1 and 19%, by weight of the compoundsfunctionalized by groups B and C—based on the polymer—are added. Throughthe amount of the copolymerized carboxylic anhydride and through theamount of the compound functionalized with B or C it is possible tocontrol the reaction rate for the polymer-analogous reaction in themelt.

[0043] In one procedure which is very favorable for the process,catalysts are added in order to raise the reactivity. The fraction ofthe catalyst is between 0.01 and 5 mol %, but preferably between 0.1 and0.5 mol %, based on the carboxylic anhydride.

[0044] The reaction proceeds under catalysis by acid or bases. As acidsit is possible to use all Lewis acid compounds. The reaction proceedspreferably with p-toluenesulfonic acid, itaconic acid, dibutyltin oxideor with sodium acetate. As bases it is possible to use all Lewis bases.The reaction proceeds preferably under 4-vinylaniline catalysis.

[0045] In accordance with the flow viscosity of the polyacrylate used,the reaction proceeds at elevated temperatures. The temperatures chosenare preferably between 60 and 180° C.: in one particularly preferredrange, between 110 and 160° C.

[0046] For the process of the invention it may likewise be of advantageto vary the molecular weight and to improve the processing properties inthe melt. Thus it is possible, for example, by reducing the molecularweight to lower the flow viscosity and so to increase the reactionpropensity. A further point is the processing properties under shear inthe extruder, since PSAs of relatively low viscosity and relatively lowmolecular mass are easier to process in the extruder and the shearintroduced is therefore greatly reduced.

[0047] Compounding—that is, the addition of further additives—maygenerally be carried out likewise in the same apparatus as the previoussteps, in a further extruder or in a compounder, where additionalcommixing of the polymer composition may also take place. To produce theadhesive tapes, the polymers described above are optionally blended withcrosslinkers: suitable crosslinker substances in this sense aredifunctional or polyfunctional acrylates, difunctional or polyfunctionalisocyanates or difunctional or polyfunctional epoxides. It is, however,also possible here to use any further difunctional or polyfunctionalcompounds which are familiar to the skilled worker and are capable ofcrosslinking polyacrylates.

[0048] For crosslinking with ultraviolet radiation, photoinitiators areused. Examples of photoinitiators that may be mentioned, without wishingto impose unnecessary restriction, include cleaving (radical-forming)photoinitiators, especially α-cleavers, and hydrogen abstractors. Forthe group of the photo cleaving initiators, examples that may bementioned include aromatic carbonyl compounds, especially benzoinderivatives, benzil ketals, and acetophenone derivatives. The hydrogenabstractors include, for example, aromatic ketones, such asbenzophenone, benzil, and thioxanthones, for example.

[0049] Moreover, in order to prepare pressure sensitive adhesives, theseelastomers are optionally blended with at least one resin. Tackifyingresins to be added include without exception all existing tackifierresins described in the literature. Representatives that may bementioned include pinene resins, indene resins, and rosins, theirdisproportionated, hydrogenated, polymerized, esterified derivatives andsalts, the aliphatic and aromatic hydrocarbon resins, terpene resins andterpene-phenolic resins, and also C5, C9 and other hydrocarbon resins.Any desired combinations of these and further resins may be used inorder to adjust the properties of the resulting adhesive in accordancewith what is desired. In general it is possible to use all resins whichare compatible (soluble) with the corresponding polyacrylate. Explicitreference is made to the depiction of the state of the art in the“Handbook of Pressure Sensitive Adhesive Technology” by Donatas Satas(van Nostrand, 1989).

[0050] The acrylic hotmelts may further be blended with one or moreadditives such as aging inhibitors, light stabilizers, ozoneprotectants, fatty acids, resins, plasticizers, nucleators, blowingagents, and accelerators. As aging inhibitors it is possible to use bothprimary and secondary aging inhibitors and also light stabilizers,including their combination with one another. Reference is made only atthis point to the appropriate Irganox™ grades from Ciba Geigy andHostanox™ from Clariant. As further outstanding agents against aging itis possible to use phenothiazine (carbon radical scavenger) and alsohydroquinone methyl ether in the presence of oxygen, and also oxygenitself.

[0051] Furthermore, the hotmelt PSAs may be filled with one or morefillers such as fibers, carbon black, zinc oxide, titanium dioxide,solid microbeads, solid or hollow glass beads, silica, silicates, andchalk, with the addition of blocking-free isocyanates being a furtherpossibility.

[0052] The acrylic PSAs blended in this way are preferably processedfurther from the melt (as hotmelts). For use as an adhesive for adhesivetapes, they are coated onto a backing and then crosslinked in order toincrease the cohesion.

[0053] It is advantageous to carry out coating of the functionalizedacrylic PSA from the melt in gel-free form. For this purpose it ispreferred to use melt dies or extrusion dies having a slot width of from100 to 500 μm, more preferably from 150 to 300 μm.

[0054] As backing materials in this context it is possible to use thematerials which are customary and familiar to the skilled worker, suchas films (polyesters, PET, PE, PP, BOPP, PVC), nonwovens, foams, wovensand woven films, and also, where appropriate, release paper (forexample, glassine, HDPE, LDPE). This list is not intended to beexclusive.

[0055] The adhesives are crosslinked with UV light or with ionizingradiation. Remaining vinyl compounds which have not undergone reactionduring the hotmelt process or reactive extrusion react with the radicalsthat are formed during crosslinking and at this point in time, at thelatest, become attached to the polymer, so that they are no longer ableto escape from the PSA tape at a later time. Crosslinking by UV rays orelectron beams proceeds with great efficiency owing to the double bondsthe side chains contain.

[0056] In comparison to non-functionalized polyacrylates, it is possibleto lower the dose required for optimum crosslinking, thereby requiringless energy and, in the case of electron beam crosslinking, causing lessdamage to the backing material. Moreover, a cohesion-enhancing effecthas been obtained.

[0057] In contrast to polyacrylates modified by allylic double bonds,there is no substantial reduction in thermal stability in the case ofthe polyacrylates produced by the inventive process. The thermalstability remains sufficiently high for processing by the hotmeltcoating process. Accordingly, all-acrylate systems prepared in this wayare gel-free for at least 48 hours at 140° C., resin-blended systems at120° C.

[0058] The process of the invention opens up application of the processof reactive extrusion for the preparation of polyacrylate-based PSAs.This result is surprising and could not have been foreseen by theskilled worker: on the contrary, the skilled worker would have expectedthe very drastic operating conditions (high temperatures, long residencetimes) typical of reactive extrusion to lead to a high level of gellingin the extruder.

[0059] Accordingly, polyacrylates prepared in the process of theinvention have incorporated into them carboxylic anhydride groups,carboxylic acid groups, and hydroxyl groups; furthermore, (meth)acrylategroups are present as side chains. Under the conditions of reactiveextrusion secondary reactions would have been expected, in the form forexample of transesterification reactions, particularly those of thepolymer chains with one another. Secondary reactions of this kind wouldresult in a high level of gelling of the polyacrylate composition.Unexpectedly, such reactions were to all intents and purposes notobserved; instead, a reaction takes place preferentially, in accordancewith the invention, between the carboxylic acid groups (preferablymaleic anhydride groups) and the functional groups B of the addedmonomers (preferably hydroxyl groups). This, surprisingly, allowspolymer-analogous reactions to be carried out in an extruder, leading tolow extruder residence times owing to the high reaction rates.

[0060] In this system, gel-free polyacrylate compositions can beprepared which have a high stability in respect of a gelling process(“gel-free” indicates compliance with the requirements for coatabilityof the compositions using the standard coating apparatus). Owing to thefreedom from gel, the polyacrylate compositions can be used foradhesives which can be coated from the melt, and can thus be used asPSAs for PSA tapes, for example. The coatability is distinguished by auniform (homogeneous) coating pattern, with no inhomogeneities, ifcoating takes place through the standard coating dies (melt dies orextrusion dies having a slot width of from 100 to 500 μm, morepreferably from 150 to 300 μm) onto, for example, polyester backingswith a thickness of 50 μm. The polyacrylate compositions commonlyprepared in reactive extrusion processes do not meet these requirementsand cannot be used as PSAs. The coating of the PSA onto a backing takesplace very preferably in an inline process, though as an alternative canalso be operated offline. After coating onto the backing, the PSA canthen be subjected to the desired crosslinking reaction.

[0061] The process of the invention provides for the first time theincorporation of (meth)acrylate groups into the side chains ofpolyacrylates in hotmelt systems. This presents the advantage of verygentle crosslinking methods, since crosslinking can be carried outdirectly by way of the incorporated acrylate groups. Where crosslinkingis carried out using electron beams, the crosslinking reaction rate isvery high and the conversion of the acrylate groups is high.Accordingly, polyacrylate PSAs prepared and crosslinked by the inventiveprocess possess very little, if any, post-crosslinking potential.Crosslinker substances which are normally added in addition aregenerally not reacted fully during the crosslinking reaction, with theconsequences that the PSAs age and the PSA products become unusable overthe course of time.

EXAMPLES

[0062] Commercially Available Chemicals Used—Trade Names ProductManufacturer Chemical composition Novares Rüttgers Aliphatic, modifiedhydrocarbon resin T K90 comprising a copolymer of unsaturated aromaticC9/C10 hydrocarbons and an aliphatically unsaturated component Softeningrange 85 to 95° C. Vazo 67 DuPont 2,2′-Azobis(2-methylbutyronitrile)Perkadox 16 Akzo Nobel Bis-(4-tert-butylcyclohexyl) peroxydicarbonateIrgacure 819 Ciba Geigy Bis(2,4,6-trimethylbenzoyl)- phenylphosphineoxide

[0063] Test Methods

[0064] The following test methods were used to evaluate the technicaladhesive properties of the PSAs prepared.

[0065] Shear Strength (Test A)

[0066] A strip 13 mm wide of the adhesive tape was applied to a smoothsteel surface which had been cleaned three times with acetone and oncewith isopropanol. The area of application measured 20 mm×13 mm(length×width).

[0067] The adhesive tape was then pressed onto the steel backing fourtimes using a 2 kg weight. At room temperature, a 1 kg weight wasfastened to the adhesive tape and the time taken for the weight to falloff was measured.

[0068] The shear stability times measured are reported in minutes andcorrespond to the average of three measurements.

[0069] Determination of the Gel Fraction (Test B)

[0070] The carefully dried, solvent-free adhesive samples are weldedinto a pouch of polyethylene nonwoven (Tyvek web). From the differencein the sample weights before and after extraction with toluene the gelindex is determined, i.e., the percentage weight fraction of the polymerthat is not soluble in toluene.

[0071] IR Spectroscopy

[0072] The FT-IR IFS 45 spectrometer from Bruker was used for themeasurement. A calibration plot was first compiled using differentconcentrations of the individual carboxylic anhydrides. The conversionof the corresponding fractions of carboxylic anhydride was determined bymeasuring the percentage fall in the CO band.

[0073] Samples Analyzed

[0074] The carboxylic anhydrides used are available commercially. 2-HEA(2-hydroxyethyl acrylate) and 2-HEMA (2-hydroxyethyl methacrylate) werepurified by distillation beforehand and stored under a nitrogenatmosphere.

Example 1

[0075] A reactor conventional for free-radical polymerizations wascharged with 500 g of 2-ethylhexyl acrylate, 350 g of methyl acrylate,70 g of butyl acrylate, 80 g of 4-methacryloyloxyethyl trimellitateanhydride and 540 g of acetone/special-boiling-point spirit (1:1).Nitrogen gas was passed through the mixture for 45 minutes, followed bydouble degassing, after which the reactor was heated to 58° C. withstirring and 0.2 g of azoisobutyronitrile (AIBN) was added. The externalheating bath was then heated to 70° C. and reaction was carried outconstantly at this external temperature. After a reaction time of 1 houra further 0.2 g of AIBN was added. After 3 hours and 6 hours, in eachcase 250 g of acetone/special-boiling-point spirit (1:1) were used fordilution. The reaction was terminated after a time of 24 hours and themixture was cooled to room temperature.

[0076] For adhesive testing, 100 g of the adhesive (based on solids)were blended with 0.4 g of bis(2,4,6-trimethylbenzoyl)phenylphosphineoxide (Irgacure 819; Ciba Geigy) and the adhesive was applied at acoverage of 50 g/m² (based on solids) to a primed PET film (23 μmthick). The specimens were then irradiated at 20 m/min (4 passes underthe lamp) using a UV irradiation unit from Eltosch (254 nm, 120 W/cm).The resulting specimens were then subjected to adhesive testing inaccordance with test methods A and B.

Example 2

[0077] A reactor conventional for free-radical polymerizations wascharged with 10 g of acrylic acid, 375 g of 2-ethylhexyl acrylate, 200 gof methyl acrylate, 375 g of butyl acrylate, 40 g of itaconic anhydrideand 290 g of acetone/special-boiling-point spirit (1:1). Nitrogen gaswas passed through the mixture for 45 minutes, followed by doubledegassing, after which the reactor was heated to 58° C. with stirringand 0.2 g of azoisobutyronitrile (AIBN) was added. The external heatingbath was then heated to 75° C. and reaction was carried out constantlyat this external temperature. After a reaction time of 1 hour a further0.2 g of AIBN was added. After 3 hours and 6 hours, in each case 250 gof acetone/special-boiling-point spirit (1:1) were used for dilution.The reaction was terminated after a time of 24 hours and the mixture wascooled to room temperature.

[0078] For adhesive testing, the adhesive was applied at a coverage of50 g/m² (based on solids) to a primed PET film (23 μm thick). Thespecimens were then irradiated with an electron beam dose of 20 kGy atan acceleration voltage of 230 kV (EBC unit from Crosslinking).

[0079] The resulting specimens were then subjected to adhesive testingin accordance with test methods A and B.

Example 3

[0080] A reactor conventional for free-radical polymerizations wascharged with 20 g of acrylic acid, 810 g of 2-ethylhexyl acrylate, 50 gof methyl acrylate, 120 g of 4-methacryloyloxyethyl trimellitateanhydride and 540 g of acetone/special-boiling-point spirit (1:1).Nitrogen gas was passed through the mixture for 45 minutes, followed bydouble degassing, after which the reactor was heated to 58° C. withstirring and 0.2 g of azoisobutyronitrile (AIBN) was added. The externalheating bath was then heated to 70° C. and reaction was carried outconstantly at this external temperature. After a reaction time of 1 houra further 0.2 g of AIBN was added. After 3 hours and 6 hours, in eachcase 250 g of acetone/special-boiling-point spirit (1:1) were used fordilution. The reaction was terminated after a time of 24 hours and themixture was cooled to room temperature.

[0081] For adhesive testing, the adhesive was applied at a coverage of50 g/m² (based on solids) to a primed PET film (23 μm thick). Thespecimens were then irradiated with an electron beam dose of 15 kGy atan acceleration voltage of 230 kV (EBC unit from Crosslinking). Theresulting specimens were then subjected to adhesive testing inaccordance with test methods A and B.

Example 4

[0082] A reactor conventional for free-radical polymerizations wascharged with 20 g of acrylic acid, 430 g of 2-ethylhexyl acrylate, 100 gof methyl acrylate, 430 g of butyl acrylate, 20 g of maleic anhydrideand 212 g of acetone/special-boiling-point spirit (1:1). Nitrogen gaswas passed through the mixture for 45 minutes, followed by doubledegassing, after which the reactor was heated to 58° C. with stirringand 0.2 g of azoisobutyronitrile (AIBN) was added. The external heatingbath was then heated to 75° C. and reaction was carried out constantlyat this external temperature. After a reaction time of 1 hour a further0.2 g of AIBN was added. After 3 hours and 6 hours, in each case 300 gof acetone/special-boiling-point spirit (1:1) were used for dilution.The reaction was terminated after a time of 24 hours and the mixture wascooled to room temperature.

[0083] For adhesive testing, the adhesive was applied at a coverage of50 g/m² (based on solids) to a primed PET film (23 μm thick). Thespecimens were then irradiated with an electron beam dose of 25 kGy atan acceleration voltage of 230 kV (EBC unit from Crosslinking).

[0084] The resulting specimens were then subjected to adhesive testingin accordance with test methods A and B.

Example 5

[0085] A reactor conventional for free-radical polymerizations wascharged with 1500 g of 2-ethylhexyl acrylate, 200 g of methyl acrylate,100 g of acrylic acid, 100 g of maleic anhydride, 100 g ofN-tert-butylacrylamide and 330 g of acetone. Nitrogen gas was passedthrough the mixture for 45 minutes, followed by double degassing, afterwhich the reactor was heated to 66° C. with stirring and 1 g of Vazo 67™(DuPont) was added. After 8 hours there was again addition of 1 g ofVazo 67™ (DuPont) and 500 g of acetone. After 24 hours and 28 hours, ineach case 2.5 g of Perkadox 16 (Akzo Nobel) were added. After 32 hours,dilution was carried out using 600 g of acetone. The reaction wasterminated after 48 hours and the mixture was cooled to roomtemperature.

[0086] For adhesive testing, the adhesive was applied at a coverage of50 g/m² (based on solids) to a primed PET film (23 μm thick). Thespecimens were then irradiated with an electron beam dose of 10 kGy atan acceleration voltage of 230 kV (EBC unit from Crosslinking).

[0087] The resulting specimens were then subjected to adhesive testingin accordance with test methods A and B.

Example 6

[0088] In comparison to Example 5, the acrylic PSA was blended with 30%by weight (based on the polymer) of hydrocarbon resin TK 90™ (Rüttgers)and used for coating. The procedure was as in Example 5. The compositionwas irradiated with an EB dose of 30 kGy with an acceleration voltage of230 kV.

[0089] Implementation of the Hotmelt Operation in a Recording Extruder:

[0090] The shearing and thermal exposure of the acrylic hotmelts wascarried out using the Rheomix 610p recording extruder from Haake. Thedrive unit used was the Rheocord RC 300p device. The instrument wascontrolled using the PolyLab System software. The extruder was chargedin each case with 52 g of the acrylic PSA/monomer mixture (˜80% filllevel). The experiments were conducted at a kneading temperature of 110or 130° C., a rotary speed of 30 rpm, and a kneading time of one hour.The specimens were subsequently coated as hotmelts through a slot die atabout 130° C.

Example 1#

[0091] In analogy to Example 1, the acrylic PSA was freed from thesolvent after cooling and 100 g of the acrylic hotmelt were mixed with1.8 g of 2-HEA (2-hydroxyethyl acrylate) and about 0.1 g of4-vinylaniline. 52 g of this mixture were processed at 110° C. in therecording extruder in accordance with the procedure set out above. Afterthe end of the reaction and after coating, the procedure of Example 1was followed.

[0092] The conversion in the reaction was measured by way of IRspectroscopy.

Example 2#

[0093] In analogy to Example 2, the acrylic PSA was freed from thesolvent after cooling and 100 g of the acrylic hotmelt were mixed with4.6 g of 2-HEMA (2-hydroxyethyl methacrylate) and about 0.1 g of4-vinylaniline. 52 g of this mixture were processed at 130° C. in therecording extruder in accordance with the procedure set out above. Afterthe end of the reaction and after coating, the procedure of Example 2was followed. The conversion in the reaction was measured by way of IRspectroscopy.

Example 3#

[0094] In analogy to Example 3, the acrylic PSA was freed from thesolvent after cooling and 100 g of the acrylic hotmelt were mixed with6.1 g of 2-HEMA (2-hydroxyethyl methacrylate) and about 0.1 g of4-vinylaniline. 52 g of this mixture were processed at 130° C. in therecording extruder in accordance with the procedure set out above. Afterthe end of the reaction and after coating, the procedure of Example 3was followed.

[0095] The conversion in the reaction was measured by way of IRspectroscopy.

Example 4#

[0096] In analogy to Example 4, the acrylic PSA was freed from thesolvent after cooling and 100 g of the acrylic hotmelt were mixed with2.6 g of 2-HEMA (2-hydroxyethyl methacrylate) and about 0.1 g of4-vinylaniline. 52 g of this mixture were processed at 130° C. in therecording extruder in accordance with the procedure set out above. Afterthe end of the reaction and after coating, the procedure of Example 4was followed.

[0097] The conversion in the reaction was measured by way of IRspectroscopy.

Example 5#

[0098] In analogy to Example 5, the acrylic PSA was freed from thesolvent after cooling and 100 g of the acrylic hotmelt were mixed with5.8 g of 2-HEMA (2-hydroxyethyl methacrylate). 52 g of this mixture wereprocessed at 150° C. for 1 minute at 70 rpm in the recording extruder inaccordance with the procedure set out above. After the end of thereaction and after coating, the procedure of Example 5 was followed.

[0099] The conversion in the reaction was measured by way of IRspectroscopy.

Example 6#

[0100] In comparison to Example 5#, the functionalized acrylic hotmeltwas blended with 30% by weight (based on the polymer) of hydrocarbonresin TK 90™ (Ruttgers) and coated as a hotmelt from the melt. Theprocedure was as in Example 5#. The composition was irradiated with anEB dose of 30 kGy with an acceleration voltage of 230 kV.

[0101] Results

[0102] To prepare the acrylic pressure sensitive adhesives, thefollowing acrylates were first of all polymerized with the comonomerconcentrations compiled in Table 1. Polymerization was carried outconventionally using AIBN in a mixture of acetone andspecial-boiling-point spirit. The individual reaction regimes have beendescribed in the section above. TABLE 1 Example AS [%] 2-EHA [%] MA [%]n-BA [%] anhydride 1 0 50 35 7  8% BSI 2 1 37.5 20 37.5  4% ISA 3 2 81 50 12% BSI 4 2 43 10 43  2% MSA 5 5 75 10 0 15% MSA

[0103] AS: acrylic acid; 2-EHA: 2-ethylhexyl acrylate; MA: methylacrylate; n-BA: n-butyl acrylate; MSA: maleic anhydride; ISA: itaconicanhydride; BSI: 4-methacryloyloxyethyl trimellitate anhydride.

[0104] In addition to their use for reactive extrusion, examples 1-5were also subjected to adhesive testing and used as references. For thispurpose the polymers were applied conventionally from solution onto aprimed polyester film 23 μm thick. After drying at 120° C. for 10minutes the applied mass of the pure adhesive was 50 g/m². After curing,the gel index of the specimens irradiated with electron beams or UVlight was measured, after which the cohesion was determined by way ofthe shear test at room temperature. Table 2 shows the results. TABLE 2Electron Shear stability beam dose UV^(a) lamp Gel index times Example[kGy] passage [%] RT, 10 N [min] 1 0 4 x^(b) 58 580 2 20 0 39 1490 3 150 30 7785 4 25 0 42 6840 5 10 0 5 5810 6 30 0 2 165

[0105] Example 1 was cured with UV light (254 nm). At a web speed of 20m/min a gel index of 58% was obtained. As a result of the relativelyhigh a polar fraction, the cohesion of this adhesive is low. Incontrast, examples 2-6 were cured with electron beams. The gel indicesmeasured lay between 2 and 42%. The shear strength as well, with ashearing weight of 10 N, was in all cases clearly below the requiredmark of 10 000 minutes for an acrylic PSA of high shear strength.Example 6 lent itself particularly poorly to crosslinking by means ofelectron beams, since electron beam crosslinking in the presence ofresins is generally less efficient.

[0106] With these results as reference, Examples 1-5 were concentrated,i.e., freed from solvent, and so prepared for the hotmelt operation. Theacrylic hotmelts were then reacted optionally with 0.1% by weight of4-vinylaniline and with different amounts of hydroxylated acrylates. Thereaction was conducted in a recording extruder, which is able to varythe degree of shear and the reaction temperature. It is also possible torecord the variation in torque. For clarity, the process parameters andthe amounts of vinyl compounds used are listed in Table 3. TABLE 3 % bywt. of 4- % by % by Rotary Reaction vinyl- wt. of wt. of speedtemperature Example aniline 2-HEA 2-HEMA [rpm] [° C.] 1# 0.1 1.8 0 30110 2# 0.1 0 4.6 30 130 3# 0.1 0 6.1 30 130 4# 0.1 0 2.7 30 130 5# 0 05.8 70 150

[0107] Example 1# was reacted with only 0.5 mole equivalent of 2-HEA. Incontrast, examples 2#-4# were admixed in each case with equimolaramounts of 2-HEMA. The catalyst selected was 4-vinylaniline, since thedouble bond is incorporated into the polyacrylate during crosslinkingand so no residual fractions of base remain as volatile fractions in thePSA. The rotary speed for mixing was 30 rpm and so was relatively low,in order to simulate low shear of the PSA. In the case of Example 5#, ahigher shear was introduced. The reaction temperature was set at 110° C.for Example 1#, since this polymer was the lowest in viscosity. Forexamples 2#-4# the reaction and extrusion temperature was 130° C., inthe case of Example 5# it was 150° C. The reaction time for allspecimens lay at 1 hour, in the case of Example 5# at 1 minute.Thereafter, the examples were firstly coated as hotmelts through a dieonto a primed polyester backing (23 μm thick) and then, depending onexample, cured in an analogy to Table 2 using UV or electron beams andsubjected to adhesive testing. The application coverage of the pureacrylic PSA was again 50 g/m². In addition, the conversion in thereaction was determined by FT-IR. The results of these tests aresummarized in Table 4. TABLE 4 Shear Electron stability Conversion beamUV^(a) Gel times of dose lamp index RT, 10 N anhydride Example [kGy]passage [%] [min] [%] 1#  0 4 x^(b) 70  4 675 38 2# 20 0 64 +10 000 803# 15 0 68 +10 000 76 4# 25 0 52 +10 000 82 5# 10 0 65 +10 000 84 6# 300 41  3 415 84

[0108] The conversion of anhydride, expressed by the percentage decreasein the CO IR band, is relatively low in the case of Example 1#, sinceonly 0.5 mole equivalent of 2-HEA was used. Nevertheless, the effect wasconsiderable for UV crosslinking. Under the same crosslinkingconditions, the gel index rose from 58 to 70%. In addition, thecomposition became significantly more cohesive after crosslinking, owingto the reactive extrusion with 2-HEA. The same trend was recorded forexamples 2#-5#. The gel indices rose, in some cases considerably, andthe shear strength of these examples was generally more than 10 000minutes. Here, a conversion of about 80% was detected by IRspectroscopy. Even the sample blended with resin shows that by reactiveextrusion it is possible to achieve a significant reduction in theelectron beam dose required.

[0109] With these examples it can be demonstrated that the principle ofreactive extrusion can be utilized for the preparation of readilycrosslinkable acrylic hotmelts. Furthermore, a cohesion-enhancing effectwas found.

[0110] As a result of the marked increased crosslinking rate it ispossible to achieve much better and more effective crosslinking,especially if acrylic and methacrylic double bonds have been introducedas crosslinking-functional groups. Even acrylic PSAs prepared by thisprocess which have a molecular weight reduced by 20-40% as compared withconventionally prepared (not functionalized) acrylic PSAs achieveadhesive properties which are just as good as those of theconventionally prepared PSAs of higher molecular weight, while owing tothe low molecular weight they have a greatly reduced viscosity andtherefore a considerably improved processing quality in the hotmeltoperation.

1. A process for preparing crosslinked acrylic pressure sensitiveadhesives which comprises first preparing polyacrylates by free-radical(co)polymerization from the following monomers: (a) acrylic andmethacrylic acid monomers of the following structure:

where R₁═H or CH₃ and R₂=an alkyl chain with 2-20 carbon atoms  with afraction of 45-99.5% by weight in the monomer mixture, (b) one or morecarboxylic anhydrides containing olefinic double bonds, with a fractionof 0.5-25% by weight in the monomer mixture, (c) further olefinicallyunsaturated monomers possessing functional groups A, with a fraction of0-30% by weight in the monomer mixture, concentrating the polymers thusprepared to give a polyacrylate composition whose solvent content is <2%by weight, adding further monomers to the polyacrylate composition,these monomers possessing at least two functional groups B and C, thegroups B being able to enter into polymer-analogous reactions with thecarboxylic anhydrides, and the groups C being crosslinkable groups, areaction taking place between the functional groups B and the carboxylicanhydride, which attaches the monomers containing functional groups B asside chains to the polymers, after the reaction between the functionalgroups B and the carboxylic anhydride, carrying out crosslinking of thepolymers by means of high-energy radiation.
 2. The process of one of thepreceding claims, characterized in that, between the reaction of thefunctional groups B with the carboxylic anhydride and the crosslinking,the pressure sensitive adhesive is applied from the melt to a backing.3. The process of one of the preceding claims, characterized in that theaddition of the monomers possessing the functional groups B and C andthe reaction of the functional groups B with the carboxylic anhydridetake place directly after the concentrating step, in particular in anextruder.
 4. The process of one of the preceding claims, characterizedin that the solvent content after concentration is ≦0.5% by weight. 5.The process of one of the preceding claims, characterized in that thefunctional groups B of the monomers are hydroxyl groups, alkoxy groups,mercapto groups, thioether groups, unsubstituted and substituted aminogroups, oxalines and/or unsubstituted or substituted amido groups. 6.The process of one of the preceding claims, characterized in that thefunctional groups C of the monomers are vinyl groups, acrylate groupsand/or methacrylate groups.
 7. The process of one of the precedingclaims, characterized in that monomers containing functional groups Band C that are used comprise 2-hydroxyethyl acrylate (2-HEA, acrylicacid 2-hydroxyethyl ester), hydroxypropyl acrylate (acrylic acid3-hydroxypropyl ester), 2-hydroxy-ethyl methacrylate (2-HEMA,methacrylic acid 2-hydroxyethyl ester) and/or hydroxypropyl methacrylate(methacrylic acid 3-hydroxypropyl ester).
 8. The process of one of thepreceding claims, characterized in that the molar ratio of the numbern_(B) of the functional groups B of the added monomers to the numbern_(CSA) of the copolymerized carboxylic anhydride units, n_(B)/n_(CSA),lies between 0.8 and 1.2, very preferably between 0.8 and
 1. 9. Theprocess of one of the preceding claims, characterized in that catalystsadded to the polyacrylate composition, especially Lewis acids,particularly p-toluenesulfonic acid, or Lewis bases, particularly4-vinylaniline.
 10. The process of one of the preceding claims,characterized in that the reaction temperature of the polymer-analogousreaction is chosen to be between 60° C. and 180° C., in particularbetween 10 and 160° C.
 11. The process of one of the preceding claims,characterized in that resins or other additives, such as aginginhibitors, light stabilizers, ozone protectants, fatty acids,plasticizers, nucleators, blowing agents, accelerators and/or fillersare added to the monomer mixture or to the acrylic pressure sensitiveadhesive.
 12. The process of one of the above claims, characterized inthat crosslinkers are added to the polyacrylate composition to becrosslinked, especially difunctional and/or polyfunctional acrylatesand/or methacrylates or photoinitiators.
 13. The process of one of thepreceding claims, characterized in that electron beams or UV radiationare used as high-energy radiation for the crosslinking.
 14. The use ofan acrylic pressure sensitive adhesive prepared by a process accordingto at least one of the preceding claims for an adhesive tape, theacrylic pressure sensitive adhesive being applied to one or both sidesof a backing.